You can't do it with a usb cable--you must buy a properly-constructed USB
device designed for this purpose--these are readily available.
You can't do it with a phone cable, unless you have installed phoneline
networking cards in each machine--these are also readily available.
You could do it with just the power system, if you installed powerline
networking cards in each system.
You can do it with an Ethernet cable, if there are Ethernet network cards in
each system AND if the Ethernet cable is properly wired as a crossover
cable--a standard cable will not work.
If you are going for lowest cost, and easiest to do, I'd go for Ethernet.
